RE: [cgmo-webcgm] ISSUE: rotation/scale center points incorrect

["Weidenbrueck, Dieter" <dweidenbrueck@ptc.com>]

Benoit,

 

you are right. Scaling always happens around the origin of the coordinate system using the 2 x 2 matrix in the upperleft of the shown 3 x 3 matrix.

 

To scale (or rotate, reflect etc) around a specific center, you have to concatenate matrices:

 

translate (-cx, -cy)

scale (sx, sy)         (or rotate(alpha) )

translate (cx, cy)

 

Dieter

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From: Bezaire, Benoit [mailto:bbezaire@ptc.com]
Sent: Donnerstag, 13. Dezember 2007 08:24
To: cgmo-webcgm@lists.oasis-open.org
Subject: [cgmo-webcgm] ISSUE: rotation/scale center points incorrect

Hi,

 

The latest proposal for the transform interface has (I believe) incorrect center points calculations.

 

The current rotation matrix:

 

[ cos(a) -sin(a) cx

  sin(a)  cos(a) cy

   0       0      1 ]

 

where a is the angle and (cx,cy) is the center point. This does not rotate a point around (cx,cy).

 

Same goes for scale matrix.

 

Benoit.